This invention relates to an electrophotographic developer.
An electrophotographic process comprises forming an electrostatic latent image on the surface of a photosensitive material, developing the image by use of a binary developer which is a mixed powder of a toner of colored resin powder and a carrier such as iron powder by triboelectrically charging the carrier and the toner, and then transferring said toner to a transfer sheet, for example, by the application of an electric field.
In recent years, in place of the above-mentioned binary developer, magnetic toner comprising a resin, and a magnetic powder, as the main constituents, has also been employed. Development using the magnetic toner is known to be advantageous in that since no carrier is needed, it is sufficient to supplement a magnetic toner consumed, so that there is neither a mixing means for triboelectrification of a carrier, a toner or a means for obtaining a uniform toner concentration by supplementing a predetermined amount of toner consumed during the development is required. Thus the developing apparatus can be small-sized.
In addition to these plain paper copier (PPC) processes permitting copying using ordinary paper, there are known coated paper copier (CPC) processes by which direct recording is conducted by using photosensitive paper. However, in the case of the latter processes, although such apparatuses are inexpensive, expensive photosensitive paper is used, so that the running cost becomes very high as compared with that of the PPC processes, and, this has led to a decrease in the use of CPC processes in recent years.
As methods of fixing these developers, there are known, for example, pressure fixing by which fixing is conducted at ordinary temperatures by using a pressure roll, radiant heat fixing by use of a halogen lamp, a nichrome wire or the like as a heat source, and contact heat fixing by use of a heat roll.
Each of these methods has both merits and demerits. Pressure fixing has merit in that quick start is possible and the consumption of electricity is small, but is disadvantageous in that it is inferior in fixing properties to the heat fixing method. Radiant heat fixing is excellent in fixing properties but low in thermal efficiency and hence disadvantageous for high-speed fixing. On the other hand, heat roll fixing has merit in that although a little time is required before a condition in which fixing is possible is obtained after the apparatus is switched on, it is excellent in fixing properties and permits high-speed fixing. But, heat roll fixing is disadvantageous in that it tends to cause a so-called offset phenomenon, i.e. a toner adhering to the surface of a heat roll once, and then adhering to a transfer sheet again.
In order to cope with this, there have been proposed, for example, a developer comprising as resin components a copolymer selected from styrene-butadiene copolymers and vinyltoluene-butadiene copolymers and polyethylene wax, as disclosed in Japanese patent application Kokai (Laid-Open) No. 130,548/80, and a developer comprising as a resin component a mixture of a partly crosslinked styrene-butadiene copolymer resin having a high molecular weight and a hydrogenated rosin ester, as disclosed in Japanese patent application Kokai (Laid-Open) No. 153,945/80.
The present inventors discovered that in the case of the developer comprising as resin components a copolymer selected from styrene-butadiene copolymers and vinyltoluene-butadiene copolymers and polyethylene wax, an image free from the offset phenomenon could be obtained by means of a heat roll whose surface had been coated with Teflon (a trade name, E.I. du Pont de Nemours & Co.) but no image sufficiently excellent in quality could be obtained. It was also found that when prepared by a grinding method, the developer showed strong elastic deformation and was very poor in productivity. On the other hand, in the case of the toner comprising as a resin component a mixture of a partly crosslinked styrene-butadiene copolymer having a high molecular weight and a hydrogenated rosin ester, said toner becomes advantageous with respect to the offset phenomenon with an increase of the proportion of crosslinking but becomes greatly disadvantageous in productivity, like the above-mentioned proposal.